Chlorosilanes such as tetrachlorosilane and trichlorosilane are produced by the reaction of raw silicon or ferrosilicon with hydrogen chloride or chlorine. After the chlorosilanes have been removed from the raw reaction product by distillation for example, there remains a residue of liquid and solid substances. This residue contains entrained metal chlorides or metal chlorides which pass over in vapor form, and which form in the reaction from the other metals contained in the raw silicon. Another portion of the residue consists of high-boiling compounds, such as titanium tetrachloride, hexachlorodisiloxane or pentachlorodisiloxane. The chief components of the residue in addition to chlorine, according to elemental analysis, are substantially silicon and aluminum. After the chlorosilanes are removed by distillation, the residue must be processed is in the form of a suspension or of a solid depending on the distillation conditions.
It is known to react the residue with steam to form hydrogen chloride (cf. DE-OS 21 61 641). A disadvantage of this method is that by the addition of a small amount of water vapor, a highly concentrated hydrogen chloride is obtained, but the remainder of the residue still contains a considerable amount of chloride which requires additional processing. By using large amounts of steam the residual chloride content can be further reduced, but the issuing hydrogen chloride contains unreacted water vapor, from which dilute hydrochloric acid forms in condensation, which is not recyclable and must be disposed of.
To avoid the formation of dilute hydrochloric acid it has been proposed to perform the hydrolysis of the residue with mixtures of water vapor and hydrogen chloride and to recirculate the unreacted water. In this process too, the residue is brought into contact with large amounts of steam in order to achieve low residual chloride contents. The energy consumption of this process is large.
The problem thus was to perform the processing of the distillation residue such that in the hydrolysis the hydrogen chloride would be produced in very concentrated form needing little energy consumption, and an inert, dumpable residue having a low chlorine content would remain.